Since she is doing this at preschool, I’d start with talking to her teacher about what you have witnessed. The teacher may have the feeling that your daughter’s bossiness is in the range that her classmates have been able to mitigate with feedback. After all, one of the points of preschool is teaching children how (and how not to) give each other feedback by giving them peers their own age to interact with. What is “being a jerk” or “getting impatient” when you’re 15 is not out of the ordinary when you only have the empathy, tact and patience of a 5 year old because you are in fact a 5 year old.
As for her relationship with her brother, it isn’t a bad idea to start teaching her now that some day she and he will both be adults together and that she’ll be wise to start encouraging him to know his own mind now. This way she won’t teach him to be the kind who either expects younger people to just do as he says or else just lets other people make all his decisions for him just because they think they know what is best. (You can spare her the knowledge that she doesn’t always know what is best, either, LOL.) They will also grow up with the mutual regard for the opinions of the other and will gradually grow to work out decisions jointly, rather than reaching adulthood without ever shedding the baggage of a pecking order that was appropriate when one was 5 and one was 3 but not when one is 28 and the other is 26.